1. Field of the Invention
The present invention relates to a polarizing device for converting a circularly polarized light beam into a radial polarized light beam, wherein the circularly polarized light beam is obtained by passing a linearly polarized light beam oscillated from a laser oscillator through a quarter-wave plate, and also relates to a laser unit using this polarizing device.
2. Description of the Related Art
In a semiconductor device fabrication process, the front side of a substantially disk-shaped semiconductor wafer is formed with a plurality of crossing division lines called streets to thereby define a plurality of partitioned regions arranged like a matrix, and a plurality of devices such as ICs and LSIs are formed in these partitioned regions. The semiconductor wafer is cut along these streets to divide the partitioned regions, thus fabricating the individual devices. Further, an optical device wafer is fabricated by forming a plurality of crossing streets on the front side of a sapphire substrate or the like to thereby define a plurality of partitioned regions and layering a gallium nitride compound semiconductor or the like in each of these partitioned regions to thereby form an optical device. Such an optical device wafer is cut along the streets into a plurality of optical devices such as light emitting diodes (LEDs) and laser diodes (LDs), which are widely used for electrical equipment. A technique for dividing a wafer such as a semiconductor wafer and an optical device wafer along the streets by applying a laser beam along the streets is disclosed in Japanese Patent Laid-open No. Hei 10-305420 and Japanese Patent No. 3408805.
Such a wafer is formed of silicon, sapphire, or silicon carbide, for example, and a laser beam having a wavelength fit for the wafer material and wafer processing conditions is suitably selected. The laser beam is converted into various polarized light beams such as a linearly polarized light beam, circularly polarized light beam, radial polarized light beam, and azimuthal polarized light beam, and any one of these polarized light beams is suitably selected according to wafer processing conditions. A laser beam oscillated from a normal laser oscillator is a substantially uniform linearly polarized light beam. The linearly polarized light beam oscillated from the laser oscillator is converted into a circularly polarized light beam by a quarter-wave plate, so as to improve the processability by the laser beam. The circularly polarized light beam is converted into a radial polarized light beam superior in focusing characteristics by a polarizing device.